The hypothesis tested in this study was that thyroid hormone levels during and after cardiopulmonary bypass (CPBP) would be a function of the mode of flow: steady state versus pulsatile. Three blood samples prior to, during, and after CPBP were used. 25 patients 12 in the pulsatile group were used. The data show that TSH increases during hypothermic hemodilutional CPBP free t4, remained constant, TBG decreased and recovered to 75% of the pre CPBP by 24 hours as did total T4. Total T8 decreased 20% of pre CPBP and remained low for the first 24 hours after operation. Albumin used as a marker of the degree of hemodilution decreased to 55% of pre CPBP levels during CPBP. Pulsatile flow had two effects only a) a transient rise in TSH levels at the mid CPBP interval and b) a lack of increase in albumin concentration after surgery. The latter effect was caused by an increased use of crystalloid solution in the initial post operative in this group. The conclusions were that total T3 remains at low levels and may contribute to low cardiac output and pulsativity per se has little effect on the concentrations of thyroid hormones.